During an intervention on an angiography system, real-time images are acquired with the aid of fluoroscopic X-ray transillumination, e.g. for the purpose of navigating the instruments (e.g. in head or heart). A frequently performed intervention on such a system is the embolization of tumors or arteriovenous malformations (AVMs), as indicated in FIG. 1.
Arteriovenous malformations (AVMs) are congenital malformations of the vascular system, frequently malformations of the vascular system of the central nervous system, the brain or the visceral cranium. In a malformation of said kind a direct connection exists between the arteries and the veins of the vascular system. This means that between the arteries and the veins there are no capillary vessels in which the actual exchange of oxygen and nutrients between the blood and the tissue takes place. One consequence of this is that the affected tissue region is not supplied with blood. Another consequence is that the pressure in the veins increases, causing them to widen and possibly leading to hemorrhaging. In this event brain hemorrhages in particular are potentially very critical.
Three methods for treating AVMs are currently available, these also being used in combination in most cases. Specifically, these are neurosurgical operations, radiotherapy and endovascular therapies. Irrespective of the way in which an AVM is treated, a precise knowledge of the location, shape and characteristics of the AVM, as well as of the detailed blood flow conditions, is essential for the planning and execution of the treatment. Both morphological information (location, size and type of the blood vessels) as well as functional time-dependent information (flow conditions) are therefore necessary.
For diagnostic purposes, computed tomography and magnetic resonance tomography in particular are possible as non-invasive imaging modalities. In addition an angiography is often performed in the interests of precise clarification and detailed treatment planning. In this case C-arm-based, temporally static and three-dimensionally spatially resolved imaging or two-dimensionally spatially resolved and one-dimensionally temporally resolved imaging are available as alternatives.
The interventional endovascular therapy takes place using fluoroscopy in the angiography laboratory. Angiographic scenes (in particular DSA scenes) through the corresponding vascular region are produced for planning and monitoring purposes. The scenes can be recorded on monoplane systems, though biplane systems are better suited, with two scenes being recorded in parallel from different angulations. The evaluation can be carried out for each of the two scenes.
It is important in this case to track the continuous progress of the embolization, in particular in order to prevent the reflux of embolic agent into unaffected vessels. Typically, such procedures are observed with the aid of subtracted recorded images in which only the differences from a specific mask image are to be seen, as shown for example in FIG. 1 in image B). An advantage in this case is that anatomical backgrounds are “subtracted away”. The progression of the embolization can also be monitored more effectively since following a reinitialization of the mask the embolic agent that has accumulated up to that point is also no longer visible in the (subtracted) subsequent recorded images.
On the other hand this can be a disadvantage, since following a reinitialization of the mask the physician can no longer recognize the already embolized regions. It is also of advantage for the physician not to see the newly embolized regions as a “growing black area”. It is important for the physician to be able to assess the progression, i.e. to recognize at a glance which region has been embolized before the others or, as the case may be, where newly injected embolic agent is currently accumulating.
Usually it is possible to employ what is referred to as a roadmap technique. To put it differently, a native mask is recorded which is subtracted from the following live X-ray images, i.e. during the intervention, in order to make changes visible thereby. If it is intended to study a continuous process, said mask has constantly to be reinitialized manually.
A determination method for a color-coded first evaluation image is known from DE 10 2007 024 450 A1. A computer receives a temporal sequence of X-ray images, to each of which an acquisition time is assigned and each of which represents a given contrast agent distribution in the examination region of an examination object at the respective acquisition time. The examination region comprises a vascular system and tissue supplied with blood via the vascular system. The computer in each case determines a characteristic value for each pixel of an evaluation image which uniquely corresponds to one of the blood vessels of the vascular system (single vessel pixel) for each of the X-ray images on the basis of the data values of the pixels of the respective X-ray image which lie in a first evaluation core that is defined by means of the respective single vessel pixel and is spatially uniform for all the X-ray images. The X-ray images and the first evaluation image correspond spatially to one another on a pixel-by-pixel basis. Based on the variation with time of the characteristic values of the respective single vessel pixel the computer determines for each single vessel pixel a characteristic time for the arrival time of the contrast agent at the respective single vessel pixel. In addition it assigns a color property that is characteristic of the respective characteristic time to each single vessel pixel and a color property that is independent of the characteristic time to every other pixel. The computer outputs the evaluation image color-coded in this way to a user. In this case a specific color coding scheme is proposed for each mask reinitialization.
There is a problem here with regard to the manual reinitialization of the masks, since the physician does not want to have constantly to perform the initialization himself/herself.
It is possible to perform a continuous reinitialization. An updated mask is automatically calculated continuously from the stream of native fluoroscopic images. For this purpose it is possible, for example, always to combine M consecutive images or to merge the images from the live stream continuously to form a mask. Said mask is further more subtracted from the live images, with the updated mask being used every K images or seconds (selectable by the user). As a result only the changes compared with the last mask image are ever visible, which means that the process can be tracked continuously.
In this case the selection of the most suitable time for a reinitialization is critical, since a reinitialization of the mask during a phase of major image changes can generate artifacts in the following recorded subtraction images.